1. Field of the Invention
The present invention relates to the structure of a downlink synchronization channel and cell search for obtaining synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) based cellular communications system.
2. Description of the Related Art
Recently, the OFDM technology has been widely applied to broadcast and mobile communication systems. The OFDM technology enables the radio communication channel to be cleared from multipath interferences, secures the orthogonality between multiple access users, and makes it possible to effectively use frequency resources. OFDM is more useful for high speed data transmission and broadband communications systems, compared to the CDMA (Code Division Multiple Access) technology. The cellular radio communications requires synchronization and cell search between the transmitter and the receiver in order to demodulate the received data and control information.
FIG. 1 illustrates the Dedicated Physical CHannel (DPCH) for data transmission applied to the WCDMA (Wideband CDMA) cellular communications standard downlink, according to the conventional art. As shown in FIG. 1, a radio frame 102 of 10 ms consists of 15 slots, each including 2560 chips, each having a length of 3.84 Mcps. The downlink synchronization and cell search (hereinafter “cell search”) is a process of detecting the frame starting point of the physical channels transmitted and cell-specific scrambling code applied to the physical channel transmission in the cell where the user exists.
In the WCDMA system, user equipment such as a mobile communication terminal obtains the synchronization of the slot timing in the first step of the cell search. In the second step, it obtains the group of cell-specific scrambling codes applied to the frame timing synchronization and corresponding cell. Next, it searches the cell-specific scrambling codes belonging to the obtained cell code group in order to obtain the cell-specific scrambling code applied to the base station.
Thus, the user equipment may demodulate the received data and control channels by obtaining the frame timing synchronization and scrambling code information of the cell to which it belongs, and consequently detect the cell IDentifier (ID) through the demodulation of Broadcasting CHannel (BCH). In the asynchronous communications system, the cell search is performed by detection of the scrambling code group, and detection of the cell-specific scrambling code as described above since there are numerous possible cell-specific scrambling codes.
FIG. 2 illustrates the frame structure in the time region in the conventional ODFM based system. In FIG. 2, a single radio frame 202 consists of L (#0, #1, #2, . . . , #(L−1)) OFDM symbols 200. The frame structure may also be described in the frequency region, as shown in FIG. 3. The OFDM technology is a multi-carrier transmission technology that enables the data and control channel information to be divided among multiple subcarriers and transmitted parallel.
FIG. 3 illustrates the frame structure of OFDM transmission signals both in the frequency and the time region.
In FIG. 3, a single OFDM symbol 300 consists of N (#0, #1, #2, . . . , #(N−1)) subcarriers 302 in the frequency region. Each subcarrier 302 carries the modulation symbol 304 of the information transmitted parallel. The OFDM symbol structure may be represented in the time region as shown in FIG. 4.
Referring to FIGS. 3 and 4, applying Inverse Fast Fourier Transform (IFFT) having a size of N to the N subcarrier symbols 302, gives the values of N samples 408 s0, s1, . . . , sN-1. The OFDM symbol 404 is produced by copying M samples (SN-M, . . . , sN-2, sN-1) 406 in the rear portion of the N samples onto the front end 410 of the OFDM symbol. The M samples portion copied onto the front end is called the Cyclic Prefix (CP) 400, and the original N samples portion is called the Useful symbol 402.
Considering the frame structure of the OFDM system described above, a similar cell search process may be applied to the OFDM based cellular radio communications system as in the WCDMA system. For example, the cell search process of the OFDM based system proposed in the article “Three-Step Cell Search Algorithm Exploiting Common Pilot Channel for OFCDM Broadband Wireless Access” by M. Tanno, H. Atarashi, K. Higuchi, and M. Sawahashi, IEICE Trans. Commun. Vol. E86-B, No. 1, January 2003 (hereinafter reference article) also consists of three steps. Namely, the same process as in the WCDMA is performed, except that the first step consists of OFDM symbol timing synchronization instead of slot timing synchronization, because the OFDM system requires the OFDM symbol as the basic unit for constituting the frame, as shown in FIG. 2.
Considering the OFDM symbol structure of FIG. 4, the OFDM symbol timing may be detected by using the fact that the samples constituting the CP 400 are the same as the M samples 406 in the rear part of the Useful symbol interval. The remaining two steps are the same as in the WCDMA system, wherein the second step is to obtain the frame timing synchronization and the scrambling code group, and the third step is to detect the cell-specific scrambling code.
However, the procedure of obtaining the synchronization in the second step is very complicated and overly time-consuming with respect to obtaining the synchronization based on obtaining both the frame synchronization and the cell code group search. This problem causes a cell search delay when the user equipment needs to be handed over to another cell. The problem of the second step is closely connected with the structure of the downlink synchronization channel. In the synchronization channel structure proposed in the reference article, the subcarriers belonging to the first OFDM symbols carry the cell group code, while the other symbols carry the cell-specific scrambling code. This frame structure requires the user equipment to detect the group containing the scrambling code applied to its cell in order to detect the frame starting point.
The problem concerning obtaining the frame synchronization may be resolved by repeating the same sequence in the time region, as in the synchronization channel structure specified in the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard, as shown in FIG. 5. The synchronization preamble OFDM symbol 500 as shown in FIG. 5 consists of three repeated sequences 502, 504 and 506 arranged at the front-most portion of the frame.
More specifically, the three sequences 502, 504 and 506 may be represented by multiplying an arbitrary complex number eiq with a size of 1 between one another. The user equipment determines the frame starting point by detecting the preamble 500. Even if the user equipment does not have the correct information on the pattern of the sequences 502, 504 and 506 in the preamble, it may obtain the frame synchronization through the preamble detected by searching out the OFDM symbol timing producing the three same sequences 502, 504 and 506. This process does not require the user equipment to know the sequences applied to the preamble, and therefore achieves more desirable frame synchronization than applying the synchronization channel structure proposed in the reference article.
The IEEE 802.16 system provides the same synchronization for the transmission signals between base stations, which uses a single OFDM symbol 500 to constitute the synchronization channel enabling the user equipment to successfully carry out the cell search, as shown in FIG. 5. However, in the asynchronous system or the synchronous system with numerous possible cell-scrambling codes, the preamble consisting of a single OFDM symbol as described above has difficulty ensuring the performance and structure for smoothly obtaining the downlink frame synchronization and the cell-specific scrambling code. Hence, the synchronization channel generally consists of two or more OFDM symbols so as to divide the process of obtaining the frame timing synchronization and cell-specific code into multiple steps as in the previous WCDMA system. In this case, the method of constituting the synchronization channel influences the performance and complexity of each step of carrying out the cell search.